This invention relates to long wave infrared radiation imaging systems and especially to such systems which are adapted to distinguish far-field point source objects from near-field background clutter.
Presently, a refracting or reflecting telescope is used for the sensing, isolation and the characterization of radiation from far-field "point" sources. The telescope is boosted aloft in a rocket, aircraft or similar vehicle. The vehicle is positioned to view a threat train which may contain incoming targets such as hostile nuclear missiles. These targets are first detected at a great distance and appear as point source images to the telescope. The field of view of the telescope is imaged onto a focal surface and scanned across that surface with the intent of observing one or more such targets in a track-while-scan mode.
The concept is applicable to both electromagnetic and acoustic radiation. Long wave infrared wave lengths are preferentially employed in this task, as radiation detection devices in this region are available and capable of detection of a small number of photons per second emitted by a warm object at distances of several hundred miles. However, the combined effects of thermal radiation and the reflection of sunlight and earthshine and albedo produce distinguishable patterns for objects or clouds of objects, as characterized by angular extent, magnitude, and wave length distribution or "color". A typical detector configuration located at the focal plane of the telescope comprises individual detector elements consisting of separate semiconductor chips mounted to circuit boards upon which the wiring, load resistors and low-noise electronic amplifier elements are also mounted. The detectors are grouped typically in a series of two to four arrays, each with its own optical bandpass filter to allow characterization of incoming radiation according to wavelength distribution. Images of far-field objects are scanned across the arrays, either by mechanical motion of the telescope or by rotation of one or more optical elements within the telescope.
The image size of a far-field point source object is, in general, limited by diffraction of the telescope optical system, which produces a characteristic blur. The size of the detector elements has been determined based upon the size of the characteristic blur. Conventionally, the along-scan dimension of the detector elements is sized to match the along scan dimension of the blur. There are two reasons for this size consideration. Detector elements of significantly greater dimension than the blur produce signals of much poorer spatial resolution so that the position uncertainty of objects increases. When the detector elements are made small with respect to the blur, less radiation is intercepted by the detector, and the signal levels increase. Inasmuch as the noise floor for signal detection is generally limited by the amplifier and not the detector, the use of smaller detectors degrades the minimum size of objects which can be sensed and the range at which the objects can be detected.
In the cross-scan direction, other criteria must be considered. In order to sense and characterize all objects in a threat train, a significant field of view must be encompassed. The detector elements must in combination sweep out this field of view. On the other hand, the number of detectors is limited by the number of electronic channels which can be crowded into the focal plane array using available technology. A further limitation is imposed by the data processing hardware which can perform the needed pulse extraction, correlation and other data processing in real-time. Accordingly, to fill the field of view in several wavelength bands, the detectors have traditionally been made long in the cross-scan direction. These considerations lead to a detector length several times the minimum resolution element. In order to allow the determination of cross-scan position to an uncertainty small with respect to detector length, the positions of the detectors are staggered in the along scan direction.
Several difficulties are inherent in these detectors. First, when two targets approach each other at the same elevation position, the signals add together on a given detector and it becomes difficult to separate their radiometric qualities and pursue their magnitude and color classification. Second, since the detectors are several times the size of the point source image blur, their response to defocused clutter signals is relatively large. Finally, when an image crosses the end of a detector, the proportion of image energy falling on or off the detector is difficult to determine, and such measurements are generally discarded in radiometric value determination.
Improvement of the state of art of focal plane detector fabrication and signal processing hardware has lead to studies, proposals, and research into focal plane assemblies with large numbers of detectors, up to the thousands or hundreds of thousands. Typically, however, these arrays, or mosaics as they are called, are laid out in orthogonal rows and columns. To overcome the problems with partial image coverage, the detector sizes are made small with respect to the image blur size, leading, with reasonable fields of view, to detector numbers in the millions. Present readout and data processing technology is not capable of handling this arrangement.
Accordingly, a need has arisen for a focal plane detector array and a processing scheme which can overcome the above-discussed deficiencies and yet is capable of operation with present processing technology.